1. Field of the Invention
This invention relates generally to the art of handling work to be processed in a series of successive processing operations performed in successive work processing stations. The invention relates more particularly to a novel work handling method and apparatus for this purpose and especially for transporting printed circuit boards through successive liquid baths of acid, rinse solution, flux, solder, and hot oil to solder coat or plate the circuit board conductors and thru-holes and then remove excess solder from the boards.
2. Discussion of the Prior Art
As will appear from the ensuing description, the work handling method and apparatus of the invention may be used for a variety of work processing applications involving movement of work to be processed through a series of successive work processing stations. The work processing operations or actions which occur or are performed in these stations can vary considerably from one application to another. A present, particularly useful application, however, involves processing printed circuit boards to coat or plate the exposed circuit board conductors and thru-holes, if any, with solder to facilitate attachment of circuit elements to the boards. For this reason, the invention will be described in the context of such circuit board processing. The term solder as used herein is intended to cover any and all conductive bonding materials which may be used for the purpose described.
Printed circuit boards are well known and widely used in the electronics industry to facilitate electrically connecting multiple electrical circuit elements, such as resistors, capacitors, transistors, integrated circuit elements, and the like in preselected electrical circuit configurations. Simply stated, a printed circuit board comprises an insulating substrate with an exposed pattern of conductors of copper or the like on at least one side to which circuit elements may be electrically connected. Many circuit boards have one or more additional levels of such conductive patterns and plated thru-holes which electrically connect the different levels in such a way as to establish the desired circuit configurations on the circuit boards.
In order to facilitate electrical connection of circuit elements to the exposed conductors of a circuit board and to electrically connect different levels, if any, of conductors in the board via circuit board thru-holes, the exposed conductors and thru-holes are coated or plated with solder. Solder coating or plating the conductors and thru-holes of a circuit board involves a series of processing steps. These are (1) acid cleaning the exposed conductors,(2) rinsing the board to remove any remaining acid,(3) applying flux to the cleaned conductor surfaces, (4) coating or plating the fluxed conductor surfaces with solder and removing excess solder from the board, and (5) washing the boards.
In the past, these solder coating or plating steps have commonly been performed in essentially independent operations involving different methods of handling the circuit boards in the different steps. In many cases, the boards are stacked between the successive operations. As a result, the existing circuit board processing operations for solder coating or plating their conductors are time consuming, costly, and otherwise ill-suited to present day circuit board mass production requirements.